The described subject matter relates to anti-reflective nanoporous silicon for photoelectrodes for efficient production of solar fuels, such as hydrogen production by photoelectrolysis of water. In recent years, there has been a great deal of excitement about hydrogen, which is a potentially high-efficiency, environmentally clean fuel. Photoelectrochemical (PEC) production of H2 at a semiconductor/electrolyte interface has drawn much attention as a viable route to direct conversion of solar energy to a storable and clean fuel. Silicon is an earth-abundant element and a promising material for PEC water electrolysis half-reaction to produce hydrogen (H2), because of appropriate conduction band edge position relative to hydrogen evolution reaction (HER) potential, E0 (H2/H2O), and small band gap (Eg=1.12 eV) to absorb most of sunlight. The p-type silicon can be used as a photocathode in a PEC cell in conjunction with an n-type photoanode, i.e., so called photochemical diodes or Z-scheme, for direct photoelectrolysis of water. However, about 25% of incident photons are reflected away at the silicon-water interface. To further maximize conversion and storage of solar energy to H2, therefore, low reflectance silicon (Si) surface is used. Thin films of SiNx and indium tin oxide (ITO) have been adopted as anti-reflection (AR) coatings on Si surface for solid-state photovoltaic (PV) devices. For photoelectrochemical generation of H2, however, the conventional AR coatings used in solid-state PV devices are difficult to implement, since the AR coatings prevent charge transfer from Si to the AR coating/electrolyte interface to produce of H2, due to poor electrical properties of AR coatings (i.e., SiNx) and/or chemical instability associated with corrosion (i.e., ITO and WO3) in contact with the aggressive aqueous electrolytes. Furthermore, such AR coatings generally prevent reflection only in a narrow band of wavelengths and for a narrow range of incident light angles, while efficient photoelectrochemistry in sunlight uses a broadband antireflection that works at all solar angles. Therefore, it is desired to develop a robust and broadband anti-reflective Si structure without heterogeneous AR coatings for reliable and efficient production of H2 at the Si/electrolyte interface by photoelectrolysis of water.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.